Rotten Den is a large cave with several rooms located west of Xedilian, very close to Inlet Camp. If entering through Deadfall, run past some statues that shoot destruction magic. M'desi's Skull for the Hill of Suicides quest is located in a large stone room in the Sanctum, at the top of a tall altar in the centre of the room. When entering the Sanctum, there are two floors with holes to drop through. For the quickest route, drop down the eastern-most hole for the second drop. Look around for a hollowed amber limb where you drop. The skull may take a while to find.